diff --git a/src/wp-includes/block-template-utils.php b/src/wp-includes/block-template-utils.php
index c883ac73306a0..1687bdae800eb 100644
--- a/src/wp-includes/block-template-utils.php
+++ b/src/wp-includes/block-template-utils.php
@@ -1270,7 +1270,7 @@ function ( $template ) {
 				$matching_registered_templates
 			);
 
-			$query_result = array_merge( $query_result, $matching_registered_templates );
+			$query_result = array_merge( $query_result, array_values( $matching_registered_templates ) );
 		}
 	}
 
diff --git a/tests/phpunit/tests/block-template.php b/tests/phpunit/tests/block-template.php
index a70f60f86cc89..f851505ca0fc0 100644
--- a/tests/phpunit/tests/block-template.php
+++ b/tests/phpunit/tests/block-template.php
@@ -655,7 +655,7 @@ public function test_get_block_templates_from_registry() {
 
 		$templates = get_block_templates();
 
-		$this->assertArrayHasKey( $template_name, $templates );
+		$this->assertContains( 'test-plugin', wp_list_pluck( $templates, 'plugin' ) );
 
 		unregister_block_template( $template_name );
 	}
diff --git a/tests/phpunit/tests/blocks/getBlockTemplates.php b/tests/phpunit/tests/blocks/getBlockTemplates.php
index 7bbb5e5b06a2f..caeae9cf466e2 100644
--- a/tests/phpunit/tests/blocks/getBlockTemplates.php
+++ b/tests/phpunit/tests/blocks/getBlockTemplates.php
@@ -275,6 +275,33 @@ public function test_get_block_templates_should_not_leak_plugin_registered_templ
 		unregister_block_template( $template_name );
 	}
 
+	/**
+	 * Plugin-registered templates must be merged into a list so that callers
+	 * can reliably access the first result by index.
+	 *
+	 * @ticket 64216
+	 *
+	 * @covers ::get_block_templates
+	 */
+	public function test_get_block_templates_should_return_a_list_when_a_plugin_registered_template_is_included() {
+		$template_name = 'test-plugin//single-post-64216';
+		$template_args = array(
+			'content'     => 'Template content',
+			'title'       => 'Test Template',
+			'description' => 'Description of test template',
+			'post_types'  => array( 'post' ),
+		);
+		register_block_template( $template_name, $template_args );
+
+		$templates = get_block_templates( array( 'slug__in' => array( 'single-post-64216' ) ) );
+
+		$this->assertTrue( array_is_list( $templates ) );
+		$this->assertCount( 1, $templates );
+		$this->assertSame( 'single-post-64216', $templates[0]->slug );
+
+		unregister_block_template( $template_name );
+	}
+
 	/**
 	 * Data provider.
 	 *